Imaging system and related techniques

ABSTRACT

A method and apparatus for imaging using a double-clad fiber is described.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit under 35 U.S.C. §119(e) ofProvisional Patent Application No. 60/585,065 filed on Jul. 2, 2004,which application is hereby incorporated herein by reference in itsentirety.

STATEMENTS REGARDING FEDERALLY SPONSORED RESEARCH

Not applicable.

FIELD OF THE INVENTION

This invention relates generally optical imaging and more particularlyto a method and apparatus for performing three-dimensional surfacemeasurements.

BACKGROUND OF THE INVENTION

As is known in the art, fiber optic endoscopy is typically conducted bytransmitting an image through an array of fibers often referred to as afiber bundle. While successful for a variety of medical and non-medicalapplications, utilization of an array of fibers to form the imageimposes constraints on the cost, diameter, and flexibility of theimaging device.

In an attempt to overcome these drawbacks, multiple approaches employinga single optical fiber have been proposed for miniature, flexibleendoscopes. For example, one technique for confocal imaging with asingle fiber has been implemented by utilizing the core of a single-modefiber as both the source and the detection apertures. Also, miniatureconfocal microscope probes and endoscopes have been constructed byadding a mechanical micro-scanner at the tip of a single-mode fiber.Another single-fiber method for miniature endoscopy, termed spectralencoding, uses a broadband light source and a diffraction grating tospectrally encode reflectance across a transverse line within the sampleas described in Tearney et al. Opt. Lett. 27: 412 (2002). Atwo-dimensional image is formed by slowly scanning this spectrallyencoded line and a three-dimensional image may be obtained by placingthe probe in the sample arm of an interferometer as described in Yelinet al. Opt. Left. 28: 2321 (2003). The core of the single-mode fiberacts as both the source and the detection apertures for all of thesetechniques.

As is also known, one important design parameter for single-fiberendoscopy is the modal profile of the optical fiber. Single-mode opticalfibers enable high resolution imaging with small and flexible imagingprobes, but suffer from relatively poor light throughput. Furthermore,the small core of the single-mode fiber acts similarly to a pinhole infree-space confocal microscopy, preventing the detection of out-of-focuslight. For endoscopic applications, this optical sectioning may not bedesirable since a large depth of field, large working distance, and widefield of view are typically preferred. For endoscopic microscopyapplications, optical sectioning may be sacrificed for increased lightthroughput.

When illuminated by coherent sources, imaging via single-mode fibersalso introduces so-called speckle noise, which significantly reduces theeffective resolution and quality of the images. Replacing thesingle-mode fiber with a relatively large diameter multi-mode opticalfiber enables higher optical throughput and decreases speckle.Unfortunately, utilization of a large diameter multi-mode fiber severelydeteriorates the system's point-spread function and prevents the use ofinterferometry for high sensitivity and three-dimensional detection.

Recently, significant progress has been made developing high power fiberlasers utilizing double-clad (also called ‘dual-clad’) optical fibers.These fibers are unique in their ability to support single modepropagation through the core with multi-mode propagation through theinner cladding.

SUMMARY OF THE INVENTION

In accordance with the present invention, a method for imaging a samplethrough an optical fiber having a core and at least one cladding regionincludes (a) transmitting a first propagating mode of light through thecore of the optical fiber toward the sample and (b) collecting scatteredlight from the sample in at least a first one of the at least onecladding regions of the optical fiber. Using the fiber's core forillumination and the inner clad for signal collection reduces imagespeckle, improves depth of field and increases signal efficiency (i.e.allows the collection of more light). Fiber core for illumination andinner clad for signal collection increases depth of field because anincrease in the diameter of the collection aperture increases the depthof field and increased diameter of collection aperture increases theamount of light that can be detected through that aperture. This ofcourse assumes that the collection aperture diameter of the innercladding is greater than that of the core. A modeling of this effect isrepresented in FIGS. 3B and 3C below

In accordance with a further aspect of the present invention, a methodfor imaging a sample through an optical fiber having a core and at leastone cladding region includes (a) transmitting a first propagating modeof light through at least one of the at least one cladding regionstoward the sample and (b) collecting scattered light from the sample ina core of the optical fiber. With this particular arrangement, atechnique in which inner cladding of a double clad fiber (or multi-cladfiber) can be used to deliver the illumination light, and the core canbe used to collect the light. The large, high numerical aperture (NA),inner clad allows for efficient coupling of illumination light that isspatially incoherent from light sources such as Halogen, Mercury orXenon lamps. This approach maintains the reduced image speckle due tothe multiple illumination angles and the large depth of field, at theexpense of a subtle drop in image resolution. The signal collectionefficiency is lower compared to the core-illumination clad-collectionscheme discussed above, but the increase in excitation light cancompensate for that by increasing the signal.

In accordance with a further aspect of the present invention, a systemfor imaging a sample includes a light source for transmitting a firstpropagating mode of light through a core of a double-clad optical fibertoward the sample and collecting scattered light from the sample in atleast a first cladding region of the double-clad optical fiber. In someembodiments, it may be desirable to collect light in both the claddingregion and the core of the fiber.

With this particular arrangement, an imaging system which utilizes thecore of a fiber for illumination and the inner clad of the fiber forsignal collection is provided. This results in a technique which reducesimage speckle and provides improved depth of field and increased signalefficiency. Using the fiber's core for illumination and the inner cladfor signal collection reduces image speckle, improves depth of field andincreases signal efficiency. It should, however, be appreciated that thedouble-clad fiber can be used by taking the opposite approach: the innerclad can be used to deliver the illumination light, and the core tocollect the light. The large, high numerical aperture (NA), inner cladallows for efficient coupling of illumination light that is spatiallyincoherent from light sources. This approach maintains the reduced imagespeckle due to the multiple illumination angles and the large depth offield, at the expense of a subtle drop in image resolution. The signalcollection efficiency is lower compared to the core-illuminationclad-collection scheme discussed earlier, but the increase in excitationlight can compensate for that by increasing the signal. In someembodiments, it may be advantageous to process information collectedthrough both the core and the cladding regions regardless of whether theillumination light, is delivered through the core or the cladding. Byseparately collecting information through both the core and thecladding, information provided from one source (e.g. thought the core)can be used to enhance the information collected from the other (e.g.through the cladding). The information collected through each channel(e.g. the core-transmit/core-collection channel, thecore-transmit/cladding-collection channel) can be processed and combinedto provide information concerning the sample which is not readilyavailable or apparent from the information collected in any singlechannel. Since the light returned through the core has higher transversespatial frequencies than light collected through the cladding, oneexample of such combination of core and cladding light would be toutilize the information obtained through the core to sharpen theinformation received through the cladding. Additionally, light detectedfrom the core is single mode which enables three-dimensional or phasesensitive information to be obtained when used in conjunction with aninterferometer. This information can be utilized to enhance theincoherent information received by the inner cladding in the form of anoverlay or pseudocolor representation of phase or three-dimensionalinformation superimposed onto the spatially incoherent informationprovided by light detected through the inner cladding.

Similarly, information collected through acladding-transmit/core-collection channel and acladding-transmit/cladding-collection channel can be processed andcombined to provide information concerning the sample which is notreadily available or apparent from the information collected in anysingle channel. In one example of such processing, image informationcollected by the core can be utilized to sharpen image informationreceived by the inner cladding.

In summary, the use of a double-clad optical fiber in an imaging systemprovides many benefits to single optical fiber based imaging. Inaddition to improved image quality, utilization of a double clad fiberenables implementation of single mode illumination with both single- andmulti-mode detection, by incorporating a detection beam-splitter withspatial filtering. This enhancement can enable dual-mode imaging wherethe multi-mode detection can be used to obtain the diffuse endoscopyimage and the single-mode detection could be used for interferometricdetection such as that employed by three-dimensional spectrally-encodedendoscopy.

It has been found that double-clad optical fiber can be used to obtainspeckle-free, signal-efficient spectrally-encoded imaging. By couplingthe illuminating broadband light into the fiber's core only, andcollecting the reflected light with the inner cladding (a configurationwhich is referred to herein as single mode-multimode or SM-MM), it ispossible to combine the benefits of single-mode illumination with theadvantages of multi-mode signal collection.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing features of this invention, as well as the inventionitself, may be more fully understood from the following description ofthe drawings in which:

FIG. 1A is a block diagram of an optical system for spectrally-encodedimaging with a double-clad fiber;

FIG. 1B is an expanded cross-sectional view of the double-clad fiber ofFIG. 1;

FIG. 2A is a photograph of a standard white light image of a face of adoll;

FIG. 2B is an image of the face of the doll of FIG. 2A generated usingspectrally-encoded imaging with single-mode illumination and single-modecollection (SM-SM);

FIG. 2C is an image of the face of the doll of FIG. 2A generated usingspectrally-encoded imaging with single-mode illumination and multi-modecollection (SM-MM);

FIG. 2D is an image of the face of the doll of FIG. 2A generated usingspectrally-encoded imaging with multi-mode illumination and multi-modecollection (MM-MM);

FIG. 3A is a block diagram of a system for signal collection using adouble-clad fiber (DCF) in which a lens images scattered light onto theface of an inner clad layer;

FIG. 3B is a plot of normalized SM-MM transverse (solid line) and axialspot sizes, plotted as a function of inner cladding diameter;

FIG. 3C is a plot of speckle contrast and normalized total signalintensity;

FIG. 4A is a block diagram of a signal collection system whichillustrates using a double-clad fiber in which a lens images scatteredlight onto the face of an inner clad layer of the fiber;

FIG. 4B is a block diagram of a signal collection system using adouble-clad fiber in which a lens images scattered light onto the faceof a core of the fiber;

FIG. 5A. is a block diagram of a system which utilizes a double-cladfiber for both coherent light collection (collection of light throughthe core only) and non-coherent light collection (light collectionthrough the inner clad only); and

FIGS. 6A-6E are a series of cross-sectional views of probes whichutilize a double clad fiber.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1, an optical system 10 for spectrally-encodedimaging with a double-clad fiber includes a broadband light source 12which transmits light to a beam splitter (BS) 14. A first portion of thelight is directed toward a double clad fiber (DCF) 16. The lightpropagates through the DCF 16 and through a collimating lens 18 to aminiature imaging probe 19.

In FIG. 1, the miniature imaging probe 19 is simulated using a compactlens grating design provided from a lens 20 and a grating 22. Agalvanometric optical scanner 24 controlled by a processor 34 toperforms slow axis scanning. The scanner directs the light toward asurface of a sample 25.

Light scattered from the sample 25 is coupled into the inner cladding orcore or both the inner cladding and core of the DCF 16 and deflected bythe beam splitter 14 (BS) to a spectrometer 29. In this exemplaryembodiment, spectrometer 29 includes a lens 28, a diffraction grating30, and a high-speed line-scan camera 32. The images can then beprovided to a computer 34 or other processing system where the imagescan be displayed and stored.

In one embodiment, the light source 12 may be provided as a broadbandlight source such as a KLM Ti:Al₂O₃ laser (750-950 nm) and coupled tothe core of the double-clad fiber 16 using an NA=0.4 microscopeobjective lens. The double-clad fiber 16 may be provided as the typeavailable through Fibercore Limited having a 3.7 μm core diameter (4.1μm mode-field diameter), NA=0.19, and a 90 μm diameter cladding, NA=0.23and identified as a SUMM900. The light transmitted from the core of theDCF 16 was collimated using an NA=0.4 microscope objective lens 18 toapproximately a 2 mm beam diameter.

Imaging using single mode detection and collection (denoted as SM-SM)was accomplished by replacing the double-clad fiber (i.e. DCF 16 inFIG. 1) and the beam splitter (i.e. BS 14 in FIG. 1) with a single-mode50/50 fiber-splitter (e.g. a single-mode 50/50 fiber-splitter of thetype provided by Gould Electronics Inc., Corning HI 780-5/125 fiber,NA=0.14). In one embodiment, the miniature imaging probe was simulatedby using a compact lens-grating design in which the beam was firstfocused by the lens 20 (L1, f=65 mm) and then diffracted by thetransmission grating 22 (G1, 1000 lines/mm) to form a line on thesurface of the sample. In another embodiment, the miniature imagingprobe comprises a DCF with an objective lens such as a gradient indexlens (GRIN) attached to the distal end of the DCF. In anotherembodiment, a spacer and angled grating is affixed to the distal end ofa GRIN lens attached to the DCF. In yet a further embodiment, the probeis comprised of a DCF, lens, dual-prism GRISM and objective lens, as isdescribed in Pitris C, Bouma B E, Shishkov M, Tearney G J. A GRISM-basedprobe for spectrally encoded confocal microscopy. Optics Express 2003;11:120-24.

The lens 28 (L2) can be provided having f=40 mm, the diffraction grating30 (G2) can be provided having 1200 lines/mm, and the camera 32 can beprovided as a high-speed line-scan camera such as a Basler L104k. Withthese components, the power on the sample was 2 mW.

To demonstrate spectrally-encoded imaging with the double-clad fiber,the face of a small doll was imaged using three different fiber-basedillumination-detection configurations. The results of these threedifferent fiber-based illumination-detection configurations are shownand described below in conjunction with FIGS. 2B-2D.

Referring now to FIG. 2A, an image of a doll's face obtained using whitelight illumination and a CCD camera is shown. The scale bar (in thelower right hand corner of FIG. 2A) represents 2 mm. This image ispresented for comparison with the images shown in FIGS. 2B-2D.

FIG. 2B is an image of the doll's face obtained with single-modeillumination and single-mode detection (denoted SM-SM).

FIG. 2C is an image of the doll's face obtained with single-modeillumination and multi-mode detection (denoted as SM-MM).

FIG. 2D is an image of the doll's face obtained using multi-modeillumination with multi-mode detection (MM-MM) where the excitationlight was coupled mainly to the inner cladding.

The SM-SM image (FIG. 2B) had relatively high resolution and contrast,but was corrupted by speckle noise. While the SM-MM image (FIG. 2C) hadslightly lower resolution compared with the SM-SM image, its appearancewas more natural and more similar to the white light reference image(FIG. 2A). Also, due to an increase in depth of field, the doll's neckand the shoulder could be seen in the SM-MM image, whereas the smallcore diameter of the SM-SM image rejected the signal coming from theseregions.

The MM-MM image (FIG. 2D) did not contain speckle noise and had thelargest depth of field, but also had a dramatically reduced resolutioncompared to the resolution of the SM-SM or SM-MM images. The images thatutilized multi-mode collection, FIGS. 2C and 2D, were also much brighterthan the SM-SM image.

In order to gain better understanding of the underlying process thatlead to these results, various imaging parameters, including thetransverse and axial resolution, collected signal intensity, and specklecontrast were numerically simulated for different inner claddingdiameters and experimentally measured for the SM-SM and SM-MMconfigurations.

Referring now to FIG. 3A, the signal collection geometry of adouble-clad fiber 41, used for the numerical simulations is shown. Thegrating G1 and the galvanometric scanner shown in FIG. 1 were omittedfrom this illustration for simplicity and to maintain the generality ofthe scheme. The spatially coherent light (dashed rays 42) emanated fromthe core 44 and was focused to a small spot on the rough surface 46 a ofthe sample 46. It was assumed that the light from the sample surface 46a scattered equally in all directions. By imaging the diffused lightspot resultant from illuminating a variety of samples e.g. paper, razorblade and a volunteer's finger, it was found that the area covered bythe diffused light extended to a typical size of about 200 mm. Thescattered light 50 (dotted lines) was imaged back onto the face of thefiber, and coupled mainly into the inner cladding 52.

For point-spread function calculations, the double-clad fiber 41 wastreated as a confocal imaging system, where the inner cladding 54 wassimulated by a finite-sized pinhole, thereby establishing acorrespondence between results obtained with the above-described systemand previously published confocal microscopy calculations. Due to thediscrete nature of the number of propagating modes in the inner cladding54, the validity of this approximation depends upon the specific fiberparameters. For example, for an NA=0.23 cladding, a 6 μm diameter fibersupports 13 propagation modes at a wavelength of 0.85 μm. This numberincreases proportionally to the cladding area, and as a result, for our90 μm diameter inner cladding, one would expect nearly 3000 modes to beguided. Since a large number of modes are guided by the inner claddingof the SMM900, the pinhole model is expected to correspond toexperimental measurements for this double-clad fiber.

Referring now to FIG. 3B, by numerically solving the Fresnel integral,the full width at half maximum (FWHM) of the point-spread function wascalculated. The transverse and the axial spot sizes, normalized to unitywhen the cladding diameter was equal to the core diameter, are shown assolid and dashed lines, respectively, in FIG. 3B. The transverse spotsize increased by up to a factor of 1.4 and then remained constant forlarge cladding diameters, while the axial spot increases almostlinearly. The transverse point-spread function was measured by takingthe derivative of the signal from an edge in an air-force resolutionchart. The full width at half maximum (FWHM) of the measuredpoint-spread function was 17.4±1.5 μm using the SM-SM configuration (themean of 35 locations on the image), and 27.7±2.9 μm for the SM-MM case(shown by a filled circle in FIG. 3B). The FWHM of the measured signal,obtained by scanning a mirror along the optical axis through the focalpoint, was measured for the SM-SM and SM-MM configurations to be about2.1±0.3 mm and 18.5±3 mm, respectively. The ratio between thesemeasurements was 8.8, which was slightly lower than the ratio of 10.5obtained from our simulation.

Efficient signal collection is important for high signal-to-noise ratioimaging. The detected signal intensity was calculated by simulating 1000rough surfaces (one random surface for each point on the sample) withuniformly distributed random amplitude and phase, within a Gaussianintensity envelope of 200 μm.

Referring now to FIG. 3C, a plot of normalized total signal intensitythat was collected with the inner cladding is shown as a dashed line. Itshould be appreciated that all SM-MM values are normalized to those ofthe SM-SM case and that the error bars represent one standard deviation.For small cladding diameters, the signal collection increased with thecladding area. The total collected signal reached a plateau as thecladding covered the entire extent of the scattered light. The totalsignal from a highly scattering paper at the object plane was measuredand it was found that the signal collected with the SMM900 innercladding was 32.5 times stronger than the signal that was collected inthe SM-SM case (diamonds in FIG. 3C). This measurement was in goodagreement with the ratio of 35 obtained from a simulation.

Speckle noise is one of the limiting factors in many coherent imagingtechniques. It reduces the effective resolution, produces imageartifacts and makes images look unnatural. Using the simulationdescribed above for the detected signal intensity, speckle noise wascalculated by dividing the standard deviation of the image by its mean.The resulting speckle contrast, plotted as a solid line in FIG. 3C,rapidly decreases with the increasing cladding diameter. The specklecontrast for 50 lines of an image of a rough aluminum surface wasmeasured. For the SM-SM configuration the speckle contrast was found tobe 0.76±0.09 and for the SM-MM case, a speckle contrast was found to be0.1±0.15 (shown in filled circles on the plot), corresponding to areduction of speckle by a factor of 7.6. This ratio was in goodagreement with that of a simulation, which demonstrated a ratio of 9.4.

These experiments and simulations show the benefits of the SM-MMconfiguration for single-fiber endoscopy. As expected, when the diameterof the inner cladding was equal to the diameter of the core (SM-SM), theresults demonstrated coherent or confocal behavior. The images in thiscase had the highest resolution and contrast, but suffered from specklenoise, low signal power and a relatively limited depth of field. TheSM-MM configuration provided by the double clad fiber is analogous toopening the pinhole in a free space confocal microscope. The large areaof the cladding improved the detection efficiency, increased the depthof field, and decreased speckle noise, resulting with natural-appearingendoscopy images.

Choosing the optimal clad diameter depends upon the requirements of thespecific application. Clad diameters around 10-20 μm, that are onlyslightly larger than the core diameter, would reduce speckle andincrease the signal with only minor reduction in both transverse anddepth resolution. Such a configuration is desired in confocal endoscopicimaging, for example, rejection of out of focus light is used to obtainoptical sectioning. When optical sectioning is not necessary, or whenlarge depth of field is required, large clad diameters can be used, aswas demonstrated in the work described above.

Double-clad optical fibers can be used to enhance several otherfiber-based imaging and non-imaging systems, in particular, systems thatdo not need coherent signal detection and would benefit from theincrease in signal and in depth of field, such as fluorescence and Ramanfiber probes.

Referring now to FIG. 4A, a system 60 for fluorescence or Raman signalcollection using a double-clad fiber probe is shown. Light 61 emanatesfrom a core 62 of a double-clad fiber 64 and is directed through a lens66 toward a surface 68 a of a sample 68. Light 69 reflects of the sample68 back through the lens 66 and onto the face of the fiber, and coupledmainly into the inner cladding 70 of the fiber 64.

Referring now to FIG. 4B, a system 60′ for fluorescence or Raman signalcollection using a double-clad fiber probe is shown. Light 61 emanatesfrom a cladding region 70′ of a double-clad fiber 64′ and is directedthrough a lens 66′ toward a surface 68 a of a sample 68. Light 69′reflects off the sample 68 back through the lens 66′ and onto the faceof the fiber, and coupled mainly into the core 62′ of the fiber 64′.

Thus, the double-clad fiber can be used by taking an approach oppositeto that described in FIG. 4A. Specifically, as shown in FIG. 4B, theinner clad can be used to deliver the illumination light, and the coreto collect the light. The large, high NA, inner clad allows forefficient coupling of illumination light that is spatially incoherentfrom light sources such as Halogen, Mercury or Xenon lamps. Thisapproach maintains the reduced image speckle due to the multipleillumination angles and the large depth of field, at the expense of asubtle drop in image resolution. The signal collection efficiency islower compared to the core-illumination clad-collection scheme discussedearlier, but the increase in excitation light can compensate for that byincreasing the signal.

Referring now to FIG. 5A, a system 72 which uses double-clad fiber (DCF)to perform both coherent and non-coherent light collection (i.e.coherent collection of light through the core only and non-coherentcollection of light through the inner clad only) includes a broadbandlight source 74 which transmits light through a fiber coupler 76 havinga first port coupled to a double-pass Rapid Scanning Optical Delay(RSOD) line and a second port coupled to a double-clad fiber (DCF) 78.Light propagates through the DCF to a sample 80. The coherent light istransmitted through the core and coupled back into a fiber splitter 82.An interference pattern between this light and the light from the delayline at the reference arm can be detected by a single detector (as shownin the figure), or by a charge coupled device (CCD) array or by usingany other technique and apparatus now known or later discovered.

It should be noted that when the DCF is used for fluorescence detection,there is no need to utilize a coherent detection scheme since thefluorescence light is not coherent. In addition to conventionalfluorescence and reflectance, other imaging modalities may benefit fromcollection of the remitted light by a second cladding of the fiber,including second harmonic, third harmonic, two-photon fluorescence,Raman scattering, coherent-anti-stokes Raman (CARS),surface-enhanced-Raman scattering (SERS) and the like.

It should be appreciated that the benefits provided by the double-cladfiber, namely the reduced speckle, the improved depth of field and theincrease in signal collection efficiency, can be obtained with differentfiber or waveguide designs. It should this be appreciated that anyconfiguration in which the sample is illuminated with a beam thatprovides a resolution spot that is acceptable by the imaging system, andthe light collection is performed by a larger aperture in the fiber, mayprovide similar benefits.

FIGS. 6A-6E are a series of cross-sectional views which illustrateseveral possible probe designs.

Referring now to FIG. 6A, a probe can be provided as a double clad fiberhaving a core 90, a first cladding layer 92 and a second cladding layer94.

Referring now to FIG. 6B, a probe can be provided as a multi-clad fiberhaving a core 96 and a plurality of cladding regions 98 a-98 c.

Referring now to FIG. 6C, a double clad fiber having a core 100 and acladding 102 with an arbitrary cladding shape is shown. It should beappreciated that while this particular embodiment is shown as a doubleclad fiber, a multi-clad fiber may also be provided a cladding layerhaving an arbitrary shape.

Referring now to FIG. 6D, a probe comprises a single-mode fiber 104 anda multimode fiber.

Referring now to FIG. 6E, a probe includes a core 108 and a plurality ofa single-mode fibers 110 a-110 f for illumination and multi modewaveguides as shown in FIG. 6E for signal collection disposed about thecore 108.

It should be understood that in addition to all of the benefits providedby the probe and fiber configurations described above, coherencedetection can still be performed by a single-mode illuminating core, orany other single mode waveguide in the probe. Coherence detection mayprovide depth sensitivity and allow for use of a heterodyne detectionscheme to allow for weak signal detection.

Although only a few exemplary embodiments of this invention have beendescribed in detail above, those skilled in the art will readilyappreciate that many modifications are possible in the exemplaryembodiments without materially departing from the novel teachings andadvantages of this invention. For example, as discussed above, using thefiber's core for illumination and the inner clad for signal collectionreduces image speckle, improves depth of field and increases signalefficiency. It should, however, be appreciated that the double-cladfiber can be used by taking the opposite approach: the inner clad can beused to deliver the illumination light, and the core to collect thelight. The large, high NA, inner clad allows for efficient coupling ofillumination light that is spatially incoherent from light sources suchas Halogen, Mercury or Xenon lamps. This approach maintains the reducedimage speckle due to the multiple illumination angles and the largedepth of field, at the expense of a subtle drop in image resolution. Thesignal collection efficiency is lower compared to the core-illuminationclad-collection scheme discussed earlier, but the increase in excitationlight can compensate for that by increasing the signal.

Accordingly, all such modifications are intended to be included withinthe scope of this invention as defined in the following claims. Itshould further be noted that any patents, applications and publicationsreferred to herein are incorporated by reference in their entirety.

1-19. (canceled)
 20. A method for imaging a sample through an opticalfiber having a core and at least one cladding region, comprising:transmitting a first propagating light through the core of the opticalfiber toward the sample; and collecting scattered light from the samplein at least a first one of the at least one cladding regions of theoptical fiber.
 21. The method of claim 20, wherein the optical fiber isprovided having a plurality of cladding regions and the method furthercomprises collecting scattered light from the sample in more than one ofthe plurality of cladding regions of the optical fiber.
 22. The methodof claim 21, further comprising collecting scattered light from thesample in each of the cladding regions of the optical fiber.
 23. Themethod of claim 22, wherein the optical fiber is provided having aplurality of cladding regions and the method further comprisescollecting scattered light from the sample in more than one of thecladding regions of the optical fiber.
 24. The method of claim 20,further comprising collecting scattered light from the sample in thecore of the optical fiber.
 25. The method of claim 20, whereintransmitting a first propagating mode of light through the core of theoptical fiber toward the sample comprises: transmitting spatiallycoherent light through the core of the fiber. focusing the spatiallycoherent light onto a spot on a surface of a sample; and collecting, inan inner cladding region of the fiber, light scattered from the samplesurface.
 26. The method of claim 25, wherein transmitting a firstpropagating mode of light through the core of the optical fiber towardthe sample further comprises focusing the spatially coherent light ontoa spot on a surface of a sample.
 27. An arrangement, comprising: a lighttransmission path arrangement which includes at least one of an opticalfiber and an optical waveguide, the light transmission is capable ofbeing propagated to illuminate the sample with a beam of light, thelight transmission path arrangement including a first fiber core; and alight collection path arrangement which is separate from the lighttransmission path arrangement, the light collection path arrangementincluding a second fiber core and a cladding region.
 28. An apparatus,comprising: a light transmission path arrangement which includes atleast one of an optical fiber and an optical waveguide, the lighttransmission is capable of being propagated to illuminate the samplewith a beam of light, the light transmission path arrangement includinga first fiber core; and a light collection path arrangement which isseparate from the light transmission path arrangement, the lightcollection path arrangement including a second fiber core and a claddingregion.
 29. An optical system for spectrally-encoded imaging,comprising: a light source; a beam splitter configured to interceptlight transmitted by the light source; a double clad fiber configured tointercept light directed thereto by the beam splitter and to allow lightto propagate along a first light transmission path thereof; acollimating lens configured to intercept light from the double cladfiber; and an imaging probe configured to receive light directed theretofrom the collimating lens and including a second light transmission pathto direct light toward the sample and a light collection path to receivelight reflected from the sample, wherein the double clad fiber includes(i) a first fiber cladding region configured to propagate the light tothe sample, and (ii) at least one of a fiber core region or a secondfiber cladding region configured to propagate the light reflected fromthe sample; the light source is a broadband light source, and theimaging probe is a miniature imaging probe; the light source transmits afirst propagating mode of spatially coherent light toward the samplethrough at least one of: a core of the optical fiber or a claddingregion of the optical fiber; the imaging probe focuses the spatiallycoherent light onto a spot on a surface of a sample; the imaging probetransmits light toward the sample through at least one of a furtherfiber core region or the first fiber cladding region; and the imagingprobe collects light reflected from the sample in at least one of: thesecond fiber cladding region of fiber or the fiber core region.